Method of producing high temperature,low pressure plasma



A ril 8, 1969 M. J. KOFOID ETAL 3,437,864

METHOD OF PRODUCING HIGH TEMPERATURE, LOW PRESSURE PLASMA Filed Aug. 29, 1966- M I /2 w 20 v I OOOOOOOOOfi INVENTOR. M. J KOFO/D 2 L Z/ES/(E ATTORNEY United States Patent US. Cl. 313-231 1 Claim This invention relates to a method for producing hot plasmas and more particularly to the generation of a hot plasma in gases at low pressures.

For many applications involving the generation of a plasma it is preferable to produce the plasma without the complicated and often disadvantageous effects caused by the presence of electrodes.

Because the coupling to the plasma load with ordinary arrangements is generally very inefficient, heating of low pressure gases by electrodeless discharges through simply the application of high frequency fields has required the use of oscillators of excessive size operating with very light loading. Such systems produce plasma Without electrodes through the employment of a high-frequency magnetic field to induce in a gas an electric field which causes current flow and concomitant heating of the gas. The apparatus and principles employed by the latter known systems are similar to those of the well-known process of induction heating of metals. Plasma torches have been used for uncontaminated heating at high temperatures in general, and more recently for generating high energy high-velocity gases for re-entry simulation and wind tunnel applications. These plasma generators have been developed to operate at powers of many kilowatts. Unlike the teachings of this invention, all of the above plasma generators must be operated at relatively high gas pressure, i.e., where the plasma producing gas is maintained at, or near, atmospheric pressure.

When the aforementioned plasma generators are operated at gas pressures as low as those used in the teachings of the instant invention, it is found in general to be impossible to obtain good coupling of load to the generator with the result there is very little heating of the gas. The plasma formed is called a cold plasma, or low power plasma, or low pressure plasma. The gas temperature of such a low pressure arc is never more than a few hundred degrees centigrade and this is the generallyheld conception where plasma is generated from gas maintained at low pressures. The temperatures achieved by prior art plasma generator devices are thus directly related to pressure. At lower pressures, it is only possible using prior art electrodeless apparatus to generate a plasma which has only relatively few electrons which transfer energy by collision to the atoms, causing emission of light, but very little heating of the gas.

According to this invention, an electrodeless inductioncoupled device is employed in a novel method for generating a hot plasma in gases maintained at low pressures; viz., at pressures less than 0.1 mm. of Hg and operational in most gases at pressures one or two orders of magnitude lower than this. More specifically, according to the teachings of this invention, a plasma can be generated from a gas medium, at a pressure as low as 0.005 mm. of Hg, having a temperature high enough to melt stainless steel or quartz in a few seconds. In addition, the teachings of this invention provide such a hot plasma at pressures where ordinary induction-coupled discharge apparatus (i.e., those not having an added steady magnetic field in parallel to the oscillating field) cease to function.

Therefore, an object of the instant invention is to provide a method of efiiciency generating a hot plasma using an electrodeless induction-coupled plasma generator.

A further object of this invention is to provide a method of efliciently generating a hot plasma in gases maintained at low pressures.

These and other objects of our invention will be apparent from the following detailed description of a preferred embodiment thereof when read in connection with the appended drawings, wherein:

FIGURE 1 represents a view in cross-section of apparatus used to practice the method of this invention.

FIGURE 2 represents a schematic view of a specific apparatus using the teachings of this invention.

Referring now to FIGURE 1, a discharge tube or chamber 10, made of a suitable electrical insulating material and having preferably an elongated cylindrical configunation, interconnects a high-pressure gas chamber 12 and a chamber 14. The chamber 12 is at a high pressure only in the sense of being relative to chamber 14; the over-all apparatus is at a pressure much lower than atmospheric pressure. The gas chamber 12, made of a strong metallic substance, is supplied with gas from a gas source 16. The chamber 14, likewise constructed of a strong metallic substance, is maintained at a lower pressure than the discharge tube 10 by a pump 18 or by simply exhausting into a materially lower pressure environment. Thus, gas admitted to tube 10 from chamber 12 is removed by Kiowing into chamber 14.

An electrical coil 20, suitably connected by circuit means 21 to a high-frequency power generator 22, is disposed concentrically about the discharge tube 10 and supported functionally by means not shown. The presence of coil 20 induces, through the presence of the changing electric current, a changing magnetic field, B-, which in turn produces a high-frequency electric field in a gas present within discharge tube 10.

A second electrical coil 24, suitably connected by circuit means 23 to a steady D-C generator source 26, is disposed concentrically about the coil 20 in order to produce a steady magnetic field B throughout the discharge tube 10. The operation of the apparatus of FIGURE 1 is dependent upon having a component of the alternating magnetic field B- parallel to a steady magnetic field B in practice the two magnetic fields B and B- will not be coincident or equal everywhere, but the degree of effectiveness of energy transfer to the plasma will be dependent: *(1) upon the magnitude of the magnetic fields B and B-; and (2) upon the degree to which the magnetic fields B and B- are parallel and substantially equal.

In operation, gas is admitted to discharge tube 10 from the higher-pressure gas chamber 12 which in turn is supplied with gas from source 16. The gas is usually continuously removed from tube 10 into a lower pressure chamber 14. The magnitude of the steady magnetic field B and the crest value of the alternating field B- within tube 10 are maintained close to equal by manipulation of power generator sources 26 and 22 respectively. More specifically, with B-=MB sin wt where M is a dimensionless numerical coefiicient, w the angular frequency of B- (to often being defined as .21rf where f is the magnetic field generator frequency), and t is time in seconds, it is preferable that M has a value of about 0.8 or 0.9. With B- in the range of several hundred (e.g., 250 to 350) guass at a frequency'of 450 'kilocycles per second, a 15 kilowatt generator 22 will be fully loaded at pressures within tube 10 of less than 0.01 mm. of Hg for both monatomic and diatomic gases. As gas is discharged through tube 10 into chamber 14, the gas at these low pressures is heated rapidly. For example, quartz tubing or rod placed concentrically within a S-centirneter diameter discharge tube 10 has been heated to a temperature of 1500 degrees centigrade with a gas plasma discharge existing for only a few seconds.

A practical application for the apparatus of FIGURE 1 includes a method for fire polishing quartz or optical lenses with ease and convenience. By disposing a lens (not shown) within tube 10 by any convenient support means (not shown), the lens can be heated very rapidly (i.e., raised to a high temperature for a short interval of time) so that only a thin surface depth of the lens reaches a state of fluidity. In this manner the surface of the lens is highly polished for optical purposes while size and shape of the lens are not altered.

Referring to FIGURE 2, a plasma formed within tube 10 in the manner described above is disposed to exhaust into a substantial lower pressure environment. In this embodiment, the teachings of this invention find particular application as a plasma gun forming a low-pressure plasma torch or application as a wind tunnel.

The invention will find use as an instrument for heating refractory or very low loss dielectric materials, or fabricated articles, for conventional or space applications with minimum contamination at lower pressures than has been heretofore efficiently accomplished. One of the drawbacks of present day wind tunnel work has been the relatively high degree of contamination within 4 the discharge nozzle in which a test article is disposed. The plasma may be produced in accordance with the teachings of this invention by using little or no gas flow through the tube 10; or the hot plasma may readily be produced where the gas is caused to flow through tube 10 at a very high rate. The invention will additionally find much use in the field of plasma chemistry.

We claim: 1. The method of efliciently producing a high temperature, low pressure plasma comprising the steps:

(a) passing a gas through a low pressure chamber;

and (b) subjecting the gas, while passing through the low pressure chamber, simultaneously to parallel and substantially equal alternating and steady magnetic fields.

References Cited UNITED STATES PATENTS 3,297,465 1/1967 Connell et al 313l6l X JAMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

U.S. Cl. X.R. 

1. THE METHOD OF EFFICIENTLY PRODUCING A HIGH TEMPERATURE, LOW PRESSURE PLASMA COMPRISING THE STEPS: (A) PASSING A GAS THROUGH A LOW PRESSURE CHAMBER; AND (B) SUBEJCTING THE GAS, WHILE PASSING THROUGH THE LOW PRESSURE CHAMBER, SIMULTANEOUSLY TO PARALLEL AND SUBSTANTIALLY EQUAL ALTERNATING AND STEADY MAGNETIC FIELDS. 